1. Field of the Invention
This invention relates to a secondary conductor supervising device of a resistance welder control system for supervising short-circuit, disconnection, and/or deterioration of a secondary conductor, inclusive of a secondary cable, of a resistance welder by means of a current flowing through the resistance welder.
2. Description of the Prior Art
The secondary conductor or secondary cable of the spot welder is subjected to expansion and bending frequently at high speed because of its property of welding work.
Recently, the case of performing such work by robots is increasing and cable wear is becoming more and more severe. To examine the extent of wear of the cable, in the prior art, a small current is caused to flow into the secondary cable by the use of a separate power source, resultant electrical values are measured by a voltmeter and an ammeter, and an increased amount of calculated resistance is inquired to determine the time of exchange of the secondary cable.
The foregoing work, however, is very laborious because some hundred secondary cables must be checked by man power. To automate such work, Japanese Utility Model Publication No. 54-37780 proposed "Secondary conductor supervising device of resistance welder". According to this proposed device, the rate of increase of impedance of the secondary cable in use as compared with that of a new secondary cable is monitored automatically, and warning is provided when the monitored rate of increase exceeds a preset level.
The conventional device of the above type operates in such a manner that it supervises deterioration of the secondary cable by means of a variation of impedance of the secondary cable. Therefore, in case the secondary cable pair has the configuration wherein its two wires are insulated from each other and stranded spirally and tightly together, like the so-called kickless type cable, the conventional device produces a comparatively minor problem. However, in case the secondary cable pair consists of two independent wires, these wires separate from each other due to a repulsive force generated upon being powered to change its impedance; thus the conventional device tends to provide warning notifying of deterioration of the cable even when the secondary cable is not disconnected nor deteriorated really. Further, the conventional device has the problem that it provides warning instantly when an accidental electrical signal caused by noise, for example, comes into its detection circuit.